The present invention relates to a shift control apparatus for an automatic transmission, and more particularly concerns a shift control apparatus to govern a shift operation from an one-way clutch (involved) gear speed, in which an oneway clutch is involved as a reaction receiving element in torque transmission, to another gear speed.
An automatic transmission is configured to determine a torque transmission path (gear speed) in a gear train by selectively hydraulically actuating (engaging) friction engagement elements such as clutches and brakes and to effect a gear change to another gear speed by switching the friction engagement elements to be engaged. The automatic transmission receives an input rotation from the engine, and delivers an output rotation at the gear ratio corresponding to a selected gear speed.
In general, a one-way clutch is used in a specific gear speed in order to prevent a shift shock in upshift, and to eliminate feel of deceleration generated by engine brake especially at a lower gear speed.
In the above-mentioned specific gear speed (referred to as an one-way clutch (involved) gear speed hereinafter), a one-way clutch is involved as a reaction receiving element in torque transmission. An upshift from the one-way clutch gear speed to a higher gear speed can be achieved only by changing the state of one friction engagement element from a disengaged state to an engaged state. In this upshift, the one-way clutch freewheels and helps prevent shift shock.
In a transition from a drive operation at the one-way clutch gear speed, to coast operation by release of the accelerator pedal, the one-way clutch disables the engine brake by freewheeling, and thereby prevents unpleasant feel of deceleration due to the engine brake at a lower gear speed.
However, the following problems has been found in a one-way clutch shift (or freewheel shift) involving the freewheeling of a one-way clutch.
In an example shown in FIG. 6, the throttle opening (degree) TVO varies with time (in a manner of time series) according to the driver""s depression of the accelerator pedal. Accordingly, a shift control system produces a 2-1 downshift command for shifting from 2nd gear speed to 1st gear speed at an instant to at which the throttle opening TVO starts to increase. Immediately afterward, at an instant t2 at which the throttle opening starts to decrease, the shift control system produces a 1-3 upshift command for shifting from 1st gear speed (one-way clutch gear speed) to 3rd gear speed (non one-way clutch gear speed). However, the shift control system inhibits the 1-3 upshift until completion of the 2-1 downshift, and allows the 1-3 upshift to start at an instant t3 at which the 2-1 downshift is finished.
During a period (a) just before instant to shown in FIG. 6, the revolution speed of an associated rotating member is held at zero by engagement of a 2nd selecting brake (a 2-4 speed brake 2-4/B in an illustrated embodiment of the present invention). The transmission is in the state of 2nd speed in which a transmission output speed No is determined by a turbine speed Nt (i.e. a transmission input speed) as shown in the speed diagram of FIG. 7A. An effective gear ratio i (i.e. a transmission input-output revolution speed ratio Nt/No) is kept at the second gear ratio as shown in FIG. 6. During this period, a one-way clutch (a low one-way clutch L/OWC in the embodiment of the present invention) functioning as a reaction receiving element in 1st speed does not participate in torque transmission as evident from the diagram.
During a period (b) just after instant to shown in FIG. 6, the 2-4 speed brake 2-4/B is disengaged and the turbine speed Nt (the transmission input speed) is increased as shown by an arrow a in FIG. 7B in response to depression of the accelerator pedal (increase of the throttle opening TVO), and the effective gear ratio i (Nt/No) is increased from the second gear ratio to the first gear ratio as shown in FIG. 6.
If the accelerator pedal was held down, the turbine speed Nt (the transmission input speed) would be further raised as shown by a two-dot chain line in the speed diagram of FIG. 7B, until the low one-way clutch L/OWC would act as a reaction receiving element to prevent reverse rotation of the associated rotating member opposite to the rotational direction of the engine, and thereby the 2-1 shift would be completed to achieve the first gear ratio.
However, in the example of FIG. 6, the accelerator pedal is released and hence the throttle opening TVO is decreased during the period (c) following the period (b), so that the turbine speed Nt (the transmission input speed) is decreased as shown by an arrow xcex2 in the speed diagram of FIG. 7C. Consequently, the low one-way clutch L/OWC becomes unable to achieve engagement as shown by an arrow xcex3, and unable to reduce the revolution speed of the associated rotating member. Thus, the transmission becomes unable to complete the 2-1 downshift, and hence the effective gear ratio i (=Nt/No) is decreased toward the third gear ratio in the direction opposite to the first gear ratio as shown in FIG. 6.
When the turbine speed Nt further decreases as shown by solid line in the speed diagram of FIG. 7D, the effective gear ratio i (=Nt/No) decreases below the third gear ratio and even nears the fourth gear ratio as shown by hatching in a period (d) in FIG. 6 in association with the transmission output speed No (vehicle speed).
When the transmission output speed No (vehicle speed) is low, the effective gear ratio i (Nt/No) does not decrease below the third gear ratio since the transmission is brought to a state shown by two-dot chain line in the speed diagram of FIG. 7D, and the output shaft speed No becomes lower than the level in the state of 3rd speed (having a gear ratio of 1) shown by one-dot chain line in FIG. 7D.
When the 2-1 downshift becomes uncompletable as described above, the shift control system forcibly terminates the control of 2-1 downshift at instant t3 at which a predetermined time has elapsed after the instant to of the 2-1 downshift command shown in FIG. 6, in order to prevent the shift control from continuing for a long time. At the same time, the shift control system permits the 1-3 upshift by canceling the reshift inhibition imposed at instant t2, and starts the 1-3 upshift operation.
Since the effective gear ratio i (=Nt/No) is decreased below the third gear ratio as shown by hatching in FIG. 6, the shift control system judges that the inertia phase of the 1-3 shift has ended, immediately after the 1-3 shift is started at instant t3. Therefore, in order to prevent the slip of the friction engagement element after engagement notwithstanding changes in the operating condition such as changes in the input torque, or to end the 1-3 shift control as soon as possible to permit a next shift to start on demand, the shift control system engages the friction engagement element for the 1-3 shift (a high-clutch H/C in the illustrated embodiment of the present invention) steeply during the short period (e) after instant t3 and thereby varies the effective gear ratio i (=Nt/No) to the third gear ratio as shown in FIG. 6. Then, the operating state is suddenly changed, as shown by an arrow xcex4, from the state shown by a solid line in FIG. 7D to the state of a one-dot chain line, so that a great shift shock.
From this reason, an undesired shift shock is caused in such a reshift control from a one-way clutch gear speed to another gear speed. A shift shock is produced also when a command from a one-way clutch gear speed to another gear speed is produced by a manual shift operation, for example while the one-way clutch free-wheels and thereby prevents the engine brake despite release of the accelerator pedal at high vehicle speeds, the engine speed is decreased by the release of the accelerator pedal, and therefore, the effective gear ratio remains at a value on the higher speed side of the after-shift gear ratio.
An object of the invention is to provide shift control apparatus or process for an automatic transmission to prevent a shift shock in a one-way clutch shift. Another object is to prevent a shift shock by monitoring a condition in a one-way clutch shift, such as an effective gear ratio or a time from a start of a shift operation to an end of an inertia phase.
According to the present invention, a shift control apparatus for an automatic transmission comprises a shift controller configured: to control a one-way clutch shift operation from a first gear ratio at which a one-way clutch is involved as a reaction receiving element for torque transmission, to a second gear ratio, normally in a normal mode; and to control the one-way clutch shift operation from the first gear ratio to the second gear ratio in a time management mode if a special condition is detected.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.